This change is far too recent for any evolutionary explanation. Rather, it seems to be a question of usage. An American anthropologist, C. Loring Brace,
put forward the thesis that the overbite results from the way we use cutlery, from childhood onwards.
What changed 250 years ago was the adoption of the knife and fork, which meant that we were cutting chewy food into small morsels before eating it.
Previously, when eating something chewy such as meat, crusty bread or hard cheese, it would have been clamped between the jaws, then sliced with a knife or
ripped with a hand -- a style of eating Professor Brace has called "stuff-and-cut."
The clincher is that the change is seen 900 years earlier in China, the reason being chopsticks.
As with any such thesis, we will probably never have definitive proof that the overbite results from the adoption of the fork, but it does seem the best
fit with the evidence.
The first time I read Brace's work, I was truly astonished. So often, we assume that the tools we use for eating are more or less irrelevant -- at most, a
question of manners. I found it remarkable that they could have this graphic impact on the human body.
At the same time, you write in the "Pots and Pans" chapter how until the 18th
century most families had one big pot, a cauldron, that had a sort of palimpsest porridge in it -- they just kept adding new things to cook along with whatever was left
over from the day before. So a lot of what people ate was soft. Were there dental changes once other ways of cooking became readily available?
The big dental change that was seen with pots happened with the initial adoption of pottery for cooking around 10, 000 years ago. Until the cooking pot was invented, no one who had lost all their teeth would survive into adulthood. There are no traces of edentulous -- toothless -- skeletons in any population without pottery. Pots made it possible for the first time to cook nourishing stew-like meals that required no chewing but could, rather, be drunk. So having teeth was no longer
necessary for survival. This is another clear example of how utensils have acted as a kind of robotic extension of the human body.
On the question of why eating these soft foods didn't lead to dramatic dental changes -- such as is seen with the fork and the overbite -- I think the
answer must be that they tended to be eaten alongside other, tougher foods.
But switching from a diet of mostly chewy foods to one of very soft foods can definitely have an impact on human teeth. Studies of Australian aboriginals
have found that within a single generation of leaving their homes in the Outback and moving to cities (where the diet becomes one heavy in refined white
flour and sugar), teeth have far less attrition (wear and tear from chewing) but many more cavities.